Mixed copper powder produced by adding graphite, or the like, to powdered copper is widely used in the manufacture of mechanical parts, such as, sintered oil-less bearings, or brushes. For example, electric brushes include a metal-graphite brush produced by powder metallurgy. The metal component mainly comprises copper to which a low-melting-point metal is added to facilitate sintering and improve corrosion resistance. Copper-coated graphite powder forms a structure having continuous copper, and is used for producing a brush having excellent sintering properties, electrical conductivity, and mechanical strength. Copper-based materials including 5-10% by weight of graphite are also used as braking friction materials.
In general, since powdered copper, or mixed powder containing copper, used in these applications will rust (be oxidized), it is treated with an organic anti-corrosion agent, such as benzotriazole, before being stored or shipped.
However, since such organic anti-corrosion agents decompose, or gasify, at temperatures above 300-400.degree. C., when the above anti-corrosion treated powdered copper or mixed powder is sintered, the anti-corrosion effect is lost, and thereafter, the sintered compact rusts (is oxidized) when it is exposed to the air similar to copper in a powdered state.
Heretofore, powdered copper, or mixed powder containing copper, has been treated only before sintering as described above with no other special treatment.
For example, Japanese Patent Laid-Open No. 5-190240 discloses a method for manufacturing an electric brush by sintering powdered electrolytic copper, ie. ultra-fine powdered copper of an average particle diameter of 15 um or less, and graphite. This reference discloses that the oxidation resistance of the brush is little affected and that the corrosion resistance effect is passive. However, the reason for the corrosion resistance effect is unclear and not disclosed.
Since the sintered compact (electric brush) of the above-referenced application has exposed copper, it is reasonable that the problem of rust (oxidation) cannot be avoided. Since an electric brush constitutes a part of an electric device or a mechanical component installed in a corrosive environment such as in a factory or, in some cases, outdoors, the problem of rust is significant.
Although non-analogous to the field of powder metallurgical technique, Japanese Patent Laid-Open No. 59-64731 discloses a technique for improving electrical conductivity, softening resistance (heat resistance), and corrosion resistance by melting and casting copper to which Pb is added. This technique is carried out by the above-described melting method, and the above-stated enhanced characteristics were achieved by alloying Pb evenly throughout the entire cast structure. However, the treatment or handling of a sintered compact made from a powdered copper, or mixed powder containing copper, or the behavior or effect of Pb in a sintered compact is not known or disclosed by the above identified application.
As described above, a consistent solution for the problem of rust (oxidation) of powdered copper or mixed powder containing copper, and the problem of the rust of sintered compacts produced therefrom is not provided by the prior art. Heretofore, since rust prevention has been applied individually by methods such as the treatment of powdered copper with an organic anti-corrosion agent, there have been problems of inefficiency and insufficient rust prevention.